When the history of the past decade of western intervention in Afghanistan is written, few will escape censure. What matters now is that there is a last chance to spare Afghanistan a fourth decade of war. More effort is being put into crafting a face-saving narrative or keeping the country out of the news altogether. The policy is called "transition" but there is little to be in transit to. Afghanistan's government preys on its people more than it serves them. The Afghan army is not what Nato claims, which is just as well, for a strong army in a fragile state is an old story that never has a happy ending. International aid to the country is roughly equivalent to its GDP, but little of this has ever reached the Afghan people, and there are commitments to reduce it gradually.

The centrepiece of this transition is next year's presidential election. It is likely to make Saturday's poll in neighbouring Pakistan, and the bloody turbulence that preceded it, seem a model of peace and propriety. At best, Afghanistan's election will lend it a semblance of self-government. At worst, it will prompt a long, violent unravelling. Washington's insurance policy is to retain special forces and the increasingly militarised CIA in the country. These partners have a long and disruptive record here, as the citizens of Wardak and Kunar provinces are only the latest to testify. If Afghanistan is treated as a no man's land, that is what it will become, and even the west's most minimal goals will not be met.

No myth about it is more self-serving than that it is ungovernable and impervious to help. What is needed is a broad and honestly brokered political process. The current approach to peace negotiations is neither. Were it to progress – it has not – it would play into the hands of Pakistan and stoke resentment among the majority of Afghans not represented. At the heart of a process involving all main Afghan constituencies must be a new constitutional assembly. Those who imposed the existing constitution on the country cannot hide the fact that it cannot work. A century of failed centralisation is enough. No less important is an accountable executive in Kabul, where the winner does not take all. For once, such discussions must happen inside the country. For this to have any hope of success, there remains a need to involve Afghanistan's near neighbours in a continuing process to replace mutual suspicion with mutual guarantees. For all the distrust and paranoia, these neighbours share vital interests. Pakistan's new prime minister, Nawaz Sharif, and a new chief of its army, will find that increased isolation is no policy for shoring up their ailing state and economy.

Both processes will need sustained mediation. The best mediators are disinterested but can call on material means of persuasion. As a party to the conflict in Afghanistan, with dysfunctional relations with the neighbours and with newer, bigger priorities, Washington must understand that it cannot play this role. Pakistan's election campaign has just emphasised the depth of anti-American sentiment. The obvious instrument of peace – the United Nations – is rusty through neglect. But it would not be so hard to sharpen it for the task. What matters is not the profile of mediators but the support of major powers, talent and impartiality. Impartiality must now be the UN's guiding principle. To be useful, it must worry more about its legitimacy in the eyes of Afghans and find a Rhadamanthine voice to answer their president's departing machinations. Afghanistan's transition is a leap in the dark. As few others will be at its side, it would be a good time and place to lavish a little attention on making an effective UN mission, not cut its modest budget. This is the sort of multilateralism that Europe, which has had little influence on Afghan policy, should unite behind.

The cost of the intervention in Afghanistan since 2001 will exceed $1tn. The human cost is incalculable. But western interests and the fate of a people still hang in the balance. If we cannot finally muster some decent strategy, there is some opiate in our culture more stupefying than anything grown in Afghanistan.